Crude oil miscibility pressure with different fluids is an important parameter to optimize for enhanced oil recovery. The “minimum miscibility pressure,” or “MMP,” is an operating parameter that is useful for the successful operation of enhanced oil recovery processes such as the injection of fluids including, but not limited to, carbon dioxide, natural gas, and nitrogen into an oil reservoir. The injection of such fluids can increase production from the treated subterranean formation by at least one of swelling the crude oil, reducing oil viscosity, and forming a mobile phase including the injected fluid and oil components.
The most common available experimental techniques to determine fluid MMP in crude oil under reservoir conditions are slim-tube displacement and pressure-composition diagrams. Among these, the slim-tube technique is presently considered the “petroleum industry standard” to determine gas-oil miscibility. In this technique, miscibility is indirectly determined from oil recovery. However, there exists no standard design, standard operating procedure, and no standard set of criteria for determining MMP using slim-tube tests. Moreover, the slim-tube test's definition of miscibility as the break-over point in the oil recovery-versus-pressure curve requires several slow-rate slim-tube displacement tests, and hence, this technique is costly and time-consuming (4-5 weeks).
Two fluids can be defined as miscible at temperature and pressure conditions where the interfacial tension between the two fluids is zero. A vanishing interfacial tension (VIT) technique has been reported (Rao, Dandina N. Fluid Phase Equilibria 1997, 139(1), 311-324) that can determine fluid MMP in crude oil. In Rao's VIT method, the gas-oil interfacial tension is measured at reservoir temperature and at varying pressures or enrichment levels of the gas phase. The gas-oil miscibility conditions are then determined by extrapolating the plot of interfacial tension against pressure or enrichment to zero interfacial tension. However, the extrapolation in Rao's VIT technique is difficult since it is performed using a curve whose slope decreases as it nears the axis with which the point of intersection must be determined. Additionally, Rao's VIT technique requires extremely accurate measurements of capillary tube inner diameter, and requires a tedious measurement of the density of each phase under each of the pressure and temperature conditions tested.